1. Field of the Invention
The present invention relates to an optical module used in an optical communications system.
2. Related Background Art
An optical module is used in a wavelength division multiplexing (WDM) transmission system using a plurality of wavelength components. In a 1.55 xcexcm band WDM system, the wavelength interval of the adjacent grids is set at approximately 0.8 nm (100 GHz), for example. In order to realize this wavelength interval, the wavelength of light emitted from the optical module must be controlled to a range of the grid wavelength plus/minus 0.03 nm.
The many of optical modules currently in use are provided with a semiconductor laser element, a photodetector, and a temperature adjuster. The wavelength of the laser light slightly emitted from a light-reflecting surface of the laser element is monitored by the photodetector, and the temperature of the laser element is adjusted based on the monitoring result to control the wavelength of the laser light. In order to detect variation of the wavelength precisely, an etalon is provided between the laser element and the photodetector. The etalon has wavelength-dependent optical transmittance. That is, the optical transmittance of the etalon is dependent on the wavelength of light incident on the etalon. Therefore, the intensity of the laser light monitored by the photodetector has a value in accordance with the wavelength thereof.
In an optical module using an etalon, the wavelength of the light emitted from the optical module is controlled in the following manner. FIGS. 1A and 1B are diagrammatic views showing relationships between the wavelength of the laser light from the laser element and the output current of the photodetector. As shown in FIG. 1A, the waveform F of the output current of the photodetector changes periodically in accordance with increases in the wavelength xcex. Here, the grid wavelength used in a WDM system is set as xcex0. The current value of the output current from the photodetector when light with wavelength xcex0 enters the photodetector is set at I0. By adjusting the temperature of the laser element so that the output current becomes the value I0, the wavelength of the laser light is locked at the grid wavelength xcex0.
The temperature of the etalon may change due to changes in usage environment and the like when the optical module is in operation. If the etalon temperature rises from T0 to T1, for example, the waveform F of the output current shifts to waveform G on the long wavelength side, as shown in FIG. 1B. This is due to thermal expansion of the etalon caused by increase in the temperature of the etalon or due to change in the refractive index of the material composing the etalon. As a result of such a wavelength shift, the lock wavelength to be locked at the grid wavelength xcex0 shifts to xcex1. When the etalon is composed of general-use, common optical glass, then the amount of the lock wavelength shift xcex94 xcex=xcex1xe2x88x92xcex0 has a temperature dependence of approximately 0.013 nm/xc2x0 C. In order to prevent variation in the lock wavelength caused by the temperature change in the etalon, the etalon is maintained at a temperature such as 25xc2x0 C., by a temperature adjuster disposed inside the optical module packaging.
The inventors have devoted themselves to research in order to enhance the wavelength controllability of an optical module, as a result of which the following findings have been obtained. The temperature of optical module packaging may rise to approximately 80xc2x0 C. depending on the environment in which the optical module is used. In such a case, a temperature difference of approximately 55xc2x0 C. occurs between the packaging and the temperature adjuster. It is proved according to the result of the research performed by the inventors that when such a temperature difference exists inside an optical module, thermal convection or thermal radiation occurs, making it difficult to maintain the temperature of the etalon at a constant level.
In light of the present situation in which the amount of information transmitted and received in optical communications systems is rapidly increasing, it is desirable to further narrow the grid wavelength interval. In order to achieve such a narrowing, the wavelength of light transmitted from an optical module must be controlled more precisely. It is therefore desirable to further suppress the variation in the lock wavelength associated with temperature change in an optical element such as an etalon.
An object of the present invention is to provide an optical module capable of suppressing lock wavelength shift caused by temperature change in an optical element.
One aspect of the present invention provides an optical module comprising: a semiconductor light-emitting device for emitting light; an optical element with wavelength-dependent optical transmittance; a photodetector which is disposed to receive light transmitted through the optical element; a temperature adjuster for heating and/or cooling the optical element; and a roof which is disposed above the optical element and thermally coupled to the temperature adjuster. The optical element has a light-receiving surface and a light-emitting surface, and is disposed to receive light from the semiconductor light-emitting device on the light-receiving surface. The temperature adjuster is disposed below the optical element.
Light emitted from the semiconductor light-emitting device passes through the optical element to enter the photodetector. The photodetector outputs an electrical signal according to the intensity of the incident light. The transmittance of the optical element is dependent on the wavelength of incident light, and therefore the light transmitted through the optical element has an intensity corresponding to its wavelength. Thus the output signal of the photodetector indicates the wavelength of the light emitted from the optical element. Accordingly, monitoring the output signal from the photodetector makes it possible to regulate the wavelength of the light emitted from the optical element to a desired lock wavelength based on the result of the monitoring. The roof is thermally coupled to the temperature adjuster and is therefore maintained at a temperature substantially equal to that of the temperature adjuster. Therefore, the temperature of the optical element, which is positioned between the roof and the temperature adjuster, is also maintained at a temperature substantially equal to those of the roof and the temperature adjuster. As a result, thermal expansion, heat shrinkage and refractive index change in the optical element are suppressed, and therefore variation in the lock wavelength is suppressed.
Another aspect of the present invention provides an optical device comprising: an optical element with wavelength-dependent optical transmittance, having a light-receiving surface and a light-emitting surface; and a holder for accommodating the optical element so that the light-receiving surface and light-emitting surface are exposed. If the optical device is installed inside the optical module so that the holder is thermally coupled to the temperature adjuster, the holder can be maintained at a temperature substantially equal to that of the temperature adjuster. Thus the temperature of the optical element accommodated in the holder is also maintained at a temperature substantially equal to those of the holder and the temperature adjuster. Since the temperature of the optical element is stabilized, variation in the lock wavelength of the optical module is suppressed.
A further aspect of the present invention provides an optical module comprising: a semiconductor light-emitting device for emitting light; an optical element with wavelength-dependent optical reflectance; a photodetector which is disposed to receive light reflected by the optical element; a temperature adjuster for heating and/or cooling the optical element; and a roof disposed above the optical element and thermally coupled to the temperature adjuster. The optical element has a light-receiving surface and is disposed to receive light from the semiconductor light-emitting device on the light-receiving surface. The temperature adjuster is disposed below the optical element.
Light emitted from the semiconductor light-emitting device is reflected by the optical element to enter the photodetector. The reflectance of the optical element is dependent on the wavelength of incident light, and therefore the light reflected by the optical element has an intensity corresponding to its wavelength. Thus the output signal from the photodetector indicates the wavelength of the light emitted from the light-emitting device. Accordingly, monitoring the output signal from the photodetector makes it possible to regulate the wavelength of the light emitted from the optical element to a desired lock wavelength based on the result of the monitoring. The roof is thermally coupled to the temperature adjuster, and therefore the roof is maintained at a temperature substantially equal to that of the temperature adjuster. Thus the temperature of the optical element, which is positioned between the roof and the temperature adjuster, is also maintained at a temperature substantially equal to those of the roof and the temperature adjuster. As a result, variation in the lock wavelength is suppressed.
A further aspect of the present invention provides an optical device comprising: an optical element with wavelength-dependent optical reflectance, having a light-receiving surface; and a holder for accommodating the optical element so that the light-receiving surface is exposed. If the optical device is installed inside the optical module so that the holder is thermally coupled to the temperature adjuster, the holder can be maintained at a temperature substantially equal to that of the temperature adjuster. Thus the temperature of the optical element accommodated in the holder is also maintained at a temperature substantially equal to those of the holder and the temperature adjuster. Since the temperature of the optical element is stabilized, variation in the lock wavelength of the optical module is suppressed.
The roof and the holder are preferably composed of a material with higher thermal conductivity than that of the optical element. In this case, the temperatures of the roof and holder exhibit good responses to the temperature change in the temperature adjuster. As a result, the lock wavelength can be even further stabilized.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.